Method of making magneto optic recording medium with silicon carbide dielectric

ABSTRACT

A magneto optic recording medium comprising a substrate, an amorphous magnetizable rare earth-transition metal alloy layer, a transparent dielectric layer on at least one side of the magnetizable layer, and a reflective surface located to reflect light through the magnetizable alloy layer. The dielectric layer is comprised of silicon carbide of the formula SiC x , wherein x, the molar ratio of carbon to silicon, is greater than 1. The dielectric layer is preferably deposited by direct current magnetron sputtering at low argon partial pressure from an electrically conductive mixture of silicon carbide and carbon. The medium exhibits similar or improved characteristics over media constructed with present dielectrics, for example, silicon suboxide (SiO y , y&lt;2).

CROSS-REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 07/150,921, filed Feb. 1,1988, now U.S. Pat. No. 4,917,970.

TECHNICAL FIELD

This invention relates to magneto optic recording media which employdielectric materials to protect a rare earth-transition metal recordingmaterial from oxidation or corrosion, enhance signal to noise ratio, actas a thermal barrier, or for other purposes. The dielectric material iscomprised of carbon-rich silicon carbide with properties desirable inthis application, such as suitable index of refraction, transparency,and ability to prevent corrosion.

BACKGROUND

Magneto optic recording media are also known by several other names:thermomagnetic media, beam addressable files, and photo-magneticmemories. All of these terms apply to a storage medium or memory elementwhich responds to radiant energy permitting the use of such energysources as laser beams for both recording and interrogation. Such mediamodify the character of an incident polarized light beam so that themodification can be detected by an electronic device such as aphotodiode.

This modification is usually a manifestation of either the Faradayeffect or the Kerr effect on polarized light. The Faraday effect is therotation of the polarization plane of polarized light which passesthrough certain magnetized media. The Kerr effect is the rotation of theplane of polarization of a light beam when it is reflected as at thesurface of certain magnetized media.

Magneto optic recording media have several advantages over knownmagnetic recording media:

1. No contact between the medium and a recording head, thus eliminatinga source of wear;

2. Using a pulsed laser beam as the writing means, very high densitydata storage is possible; and

3. With a protective layer on top of a magneto optic layer, the mediumis affected less by dust than magnetic media.

In magneto optic recording, data is written into a medium having apreferentially directed remanent magnetization by exposing a localizedarea (spot or bit) on the recording medium to an electromagnetic orother energy source of sufficient intensity to heat the recording mediumabove its Curie or compensation point temperature and simultaneouslybiasing the medium with a magnetic field. Preferably, the energy sourceis a laser which produces a monochromatic output beam. The magneticfield required to reverse the magnetization of the recording mediumvaries with the temperature to which the recording medium is brought.Generally speaking for a given material, the higher the temperature, thesmaller the required magnetic field coercive force.

The write or record operation for both Curie point and compensationpoint writing is as follows:

1. The medium is initially in a randomly magnetized state. A domain willherein refer to the smallest stable magnetizable region; although incommon usage, a domain is a uniformly magnetized region of any size. Aselected area of the medium may be magnetized by exposing it to acontinuous energy beam and a small magnetic bias field normal to thesurface of the medium.

2. A small magnetic bias field oriented perpendicular to the surface orplane of the medium, but oppositely directed to the magnetic fieldapplied earlier is applied over the entire thin film medium.

3. With the biasing field in place, a light beam from a radiant energysource such as a laser beam is directed toward a selected location orbit on the medium where it causes localized heating of the medium to atemperature at or above the Curie and/or compensation temperature. Whenthe laser beam is removed, the bit cools in the presence of the biasingmagnetic field and has its magnetization switched to that direction. Themedium, in effect, has a magnetic switching field which is temperaturedependent. The magnetic biasing field applied to the irradiated bitselectively switches the bit magnetization, with the bit momentarilynear its Curie and/or compensation temperature under the influence ofthe laser. The momentary temperature rise reduces the bit coerciveforce.

In the write operation, the write laser beam is focused to the desireddiameter (e.g. 1.0 micrometer) onto the surface of the recording mediumby an objective lens.

The memory element or recorded bit is interrogated, or read,nondestructively by passing a low-power (e.g. 1-3 mW) beam of polarizedlight (e.g. a laser beam) through the bit storage site for asufficiently short time so as not to heat the medium to change itsmagnetic state. The read laser beam is normally shaped to a circularcross section by a prism, polarized and focused to the same diameter asthe write beam onto the recording medium by a lens. When the read beamhas passed through the recorded spot, it is sent through an opticalanalyzer, and then a detector such as a photodiode, for detection of anychange or lack of change in the polarization.

A change in orientation of polarization of the light is caused by themagneto-optical properties of the material in the bit or site. Thus, theKerr effect, Faraday effect, or a combination of these two, is used toeffect the change in the plane of light polarization. The plane ofpolarization of the transmitted or reflected light beam is rotatedthrough the characteristic rotation angle θ. For upward bitmagnetization, it rotates θ degrees and for downward magnetization -θdegrees. The recorded data, usually in digital form represented by logicvalues of 1 or 0 depending on the direction of bit magnetization, aredetected by reading the change in the intensity of light passing throughor reflected from the individual bits, the intensity being responsive tothe quantity of light which is rotated and the rotation angle.

It was previously believed that the signal-to-noise ratio (SNR) orcarrier-to-noise ratio (CNR) of an erasable magneto optic medium isproportional to θ×R^(1/2) where θ is the angle of rotation and R is thereflectivity of the medium. Presently, the relationship between CNR andthe parameters of a fully constructed magneto optic medium is not wellunderstood. The process of optimizing media construction appears to bemore complicated than simply optimizing θ×R^(1/2).

Forty-five decibels in a 30 kHz band width is generally considered theminimum CNR acceptable for direct read after write (DRAW) media. Thespeed at which the bits can be interrogated and the reliability withwhich the data can be read depends upon the magnitude of the magnetooptical properties, such as the angle of rotation, and upon the abilityof the interrogation system to detect these properties. For purposes ofthis discussion, the noise floor or noise level is measured at theaverage noise level.

The main parameters that characterize a magneto optic material are theangle of rotation, the coercive force, the Curie temperature and thecompensation point temperature. The medium is generally comprised of asingle layer or multiple layer system where at least one of the layersis a thin film metal alloy composition. Binary and ternary compositionsare particularly suitable for amorphous metal alloy formation. Suitableexamples would be rare earth-transition metal (RE-TM) compositions, suchas: gadolinium-cobalt (Gd--Co), gadolinium-iron (Gd--Fe), terbium-iron(Tb--Fe), dysprosium-iron (Dy--Fe), Gd--Tb--Fe, Tb--Dy--Fe, Tb--Fe--Co,terbium-iron-chromium (Tb--Fe--Cr), gadolinium-iron-bismuth(Gd--Fe--Bi), gadolinium-iron-tin (Gd--Fe--Sn), Gd--Fe--Co, Gd--Co--Bi,and Gd--Dy--Fe.

Many of the elements which are suitable for the rare earth-transitionmetal alloy layer react strongly with oxygen and other elements whichmay be present in the environment in which the media are used.Furthermore, the substrate upon which the alloy layer is deposited mayitself contain impurities which react with the alloy layer. Thus,materials are deposited on one or both sides of the RE-TM thin film toprotect it. To be effective, such materials must not themselves reactwith the rare earth-transition metal layer or any other layer, mustoffer chemical and physical resistance to degradation by heat, humidity,and corrosive chemicals, and must be transparent at the wavelengths usedfor reading and writing of data (typically about 8200 or 8300 angstromsfor a laser diode, or approximately 6328 angstroms for a helium-neonlaser, although other wavelengths may be used). A material is"transparent" for the purposes of this discussion when it absorbs lessthan about 20 percent of the intensity of an incident light beam at aparticular wavelength.

Presently used dielectrics include silicon suboxide (SiO_(y), y<2),titanium dioxide, silicon dioxide, cerium oxide, aluminum oxide, andaluminum nitride. Most of these materials contain oxygen, which canreact with the rare earth element in the magnetizable layer and therebydegrade media performance. All these materials are dielectrics, i.e.,they have very low electrical conductivity. This prevents the use of DCmagnetron sputtering to deposit them on the other layers of a completemagneto optic medium. Instead, radio frequency (RF) sputtering,evaporation deposition, or reactive sputtering deposition, can be used.

DISCLOSURE OF INVENTION

The invention is a magneto-optic recording medium comprising asubstrate, an amorphous magnetizable rare earth-transition metal alloylayer having a transparent dielectric layer on at least one side, and areflective surface located to reflect light through the magnetizablealloy layer, wherein the dielectric layer is comprised of siliconcarbide of the formula SiC_(x), wherein x, the molar ratio of carbon tosilicon, is greater than 1.

Many substrates can be used. They may be formed of any material which isnonmagnetic, dimensionally stable, minimizing radial displacementvariations during recording and playback. Semiconductors, insulators, ormetals can be used. Suitable substrates include glass, spinel, quartz,sapphire, aluminum oxide, metals such as aluminum and copper, andpolymers such as polymethyl-methacrylate (PMMA) and polyester. Glass ispreferred for applications requiring high dimensional stability, whilepolymers are preferred for mass production due to their comparativelylower cost.

The substrate is typically a disc. Common diameters include 3.5 inches(8.9 centimeters) and 5.25 inches (13.3 centimeters), although othersizes are used. Transparent substrates allow the construction of mediain which the read and write light beams pass through the substratebefore the recording layer, then onto a reflector layer, and back againto the recording layer after reflection. Such a medium is known as asubstrate incident medium. When the reflector layer is between thesubstrate and the recording layer, the read and write beams will not bedirected through the substrate. Such a medium is known as an airincident medium, although generally there is at least one layer betweenthe recording medium and the air.

When a magnetizable amorphous material is deposited on a reflector, itis known that the magneto optic rotation is increased because theFaraday effect is added to the Kerr effect. The former effect rotatesthe plane of polarization of the light as it passes back and forththrough the magneto optic layer while the Kerr effect rotates it at thesurface of the layer. The reflective surface may be a smooth, highlypolished surface of the substrate itself, or it may be the surface of aseparate reflecting layer deposited by techniques known in the art suchas vacuum vapor deposition. The reflective surface or layer usually hasa reflectivity greater than about 50 percent (preferably 70 percent) atthe recording wavelength. Deposited reflecting layers usually are about500 to 5000 angstroms thick. Typical reflective surfaces or layers arecopper, aluminum, or gold.

The recording medium thin film typically comprises an alloy of at leastone rare earth element and at least one transition metal and usually isno more than 400 angstroms thick if a reflector is employed; if not, thefilm may need to be as thick as 2000 angstroms to produce the samemagneto optic effects, as the Faraday effect will not be present. If itis too thin, generally 50 angstroms or less, the magneto optic film maynot absorb enough light in the write mode. Sufficient coercivity tocreate a stable memory should be about 500 Oersteds (Oe), but a range of2000 to 3000 Oe is generally used.

Oxidation of the magnetizable RE-TM layer is believed to be a majorcause of loss of media performance.

A transparent layer can be deposited on one or both sides of themagnetizable amorphous film. When it is located between the reflectinglayer and the magnetizable amorphous film, it is known as theintermediate layer. In this position, dielectric materials arepreferred, as they are known to protect the alloy layer from reactingwith the reflecting layer or any impurities in it. A dielectric layeralso provides a thermal barrier, reducing heat conduction from themagnetizable amorphous film to the reflector layer, thereby reducing thelaser power required to write data in the magnetizable amorphous film.The intermediate layer is generally 0 to 300 nanometers thick. Such anintermediate layer should have an index of refraction greater than about1.2, preferably between 2.0 and 3.0. An intermediate layer with a highindex of refraction allows the magneto optic rotation angle to besignificantly increased by interference enhancement.

Interference enhancement may also occur when a second transparent layeris deposited on the other side of the magnetizable amorphous thin film.Such a layer will be called a barrier layer. Media having oneinterference layer (either an intermediate or barrier layer) plus the MOand reflective layers are referred to as trilayer media. Media havingboth an intermediate layer and a barrier layer are called quadrilayermedia. When the barrier layer is constructed from a dielectric material,it also is characterized by an index of refraction greater than 1.2,although it need not be the exact same material as the intermediatelayer. The index of refraction should not be so high, however, as toproduce too much reflection from the interface of the barrier andsubstrate layers, if the barrier layer is located adjacent a transparentsubstrate on which the polarized light is first incident (substrateincident structure). The barrier layer is usually between about 30 and200 nanometers thick.

In cases where the dielectric layer is in between the recording film andthe reflecting layer or surface and there is no barrier layer (trilayerconstruction), it is beneficial to add a transparent passivating layerover the recording film layer. Passivation is the change of a chemicallyactive metal surface to a much less reactive state. The transparentpassivating layer is basically the same as the previously describedtransparent dielectric barrier layer but thinner, typically up to about100 angstroms thick. As in the case of the transparent dielectric layeron the other side of the MO film, the passivating layer must protect therecording film from oxidation or corrosion due to excessive heat,humidity, or chemical reaction with impurities. It need not obtain thesame optical effects (e.g., θ enhancement) as the thicker barrier layer.It is possible to combine the functions of the barrier layer and thepassivating layer into a single layer comprised of a transparentdielectric material, and selecting the thickness to provide interferenceenhancement. Such a layer is still known as a barrier layer.

BRIEF DESCRIPTION OF THE DRAWING

FIG. No. 1 is an Auger spectrum of a silicon carbide layer of x=1.47 ofthis invention.

FIG. Nos. 2 and 3 are transmission electron microscope views at 200,000×of recording media of this invention, in cross section, showing thesilicon carbide dielectric layers of this invention.

DETAILED DESCRIPTION OF THE INVENTION

The magneto optic amorphous thin films can be made by known thin filmdeposition techniques, such as sputtering, evaporation and splatcooling. One preferred process is cathodic sputtering. Typical knownsputtering conditions for amorphous thin films are: initial vacuum lessthan 1×10⁻⁵ torr; sputtering pressure of from 3×10⁻² to 2×10⁻² torr;pre-sputtering of a sputtering source of material to clear the surfacethereof; substrate temperature of 30 degrees to 100 degrees C.; and anoble gas (usually argon) partial pressure.

In the cathodic sputtering process, gas ions bombard the solid alloytarget cathode in the sputtering chamber, dislodging metal atoms bytransferring the momentum of the accelerated ions to the metal atomsnear the surface of the target. The substrate is placed at the anode,and the metal alloy atoms traverse the space between the anode andcathode to deposit on the substrate.

In the triode sputtering process, a thermionic cathode, known as anemitter, is added to the sputtering chamber between the anode andcathode. This allows the gas plasma to be maintained at much lowerpressures than a direct current glow discharge, even in a magnetic fieldor magnetron. Typically an argon plasma can be maintained at 4×10⁻³ to6×10⁻⁴ torr gauge pressure. This process enables the sputtered atoms tostrike the substrate at a higher energy than they would at a higherpressure since there are fewer argon ions in the space between thetarget and substrate to interfere with the motion of the sputteredatoms, increasing the mean-free-path.

Magnetically enhanced sputtering, in which a magnetic field is appliedin the sputtering chamber perpendicularly to the electric field, furtherreduces the pressures needed to sputter and thereby increases themean-free-path. This is because the magnetic field deflects electronsinto following spiral-like paths with greater distances to travel toreach the anode. The longer path increases the probability of collisionwith the gas atoms. These collisions produce the gas ions which dislodgethe sputter target atoms, hence an increase in probability of gascollision increases the sputtering rate. Another feature of magneticenhancement is that electrons are confined to the ionized gas plasma andproduce less heating of the substrate by electron bombardment. Thisfeature is a benefit when it is desired to use substrates withcomparatively low melting points.

Currently used dielectric materials are deposited by several methodswhich have disadvantages. Evaporation deposition requires much lowervacuum pressures than the sputtering techniques used to deposit theRE-TM alloy films; uniform film deposition over a wide area isdifficult; and the deposition rate is difficult to control because it isan exponential function of evaporation boat temperature. Radio frequency(RF) sputtering produces excessive heat at the substrate, making someinexpensive plastic substrates unusable; and the RF signals caninterfere with computer controlled manufacturing processes.

Reactive sputter deposition requires the addition of a reactive gas tothe chamber, which can contaminate the other deposition processes; andthe gas may react at the sputter target surface, forming a nonconductivefilm which interferes with the sputtering.

Direct current (DC) magnetron sputtering offers superior deposition ratecontrol and deposited film thickness when compared to evaporationtechniques. Nonreactive DC magnetron sputtering (i.e., no reactive gasespresent) reduces the contamination of other processes carried out in thesame deposition chamber. Temperatures generated at the substrate are lowenough to permit the use of plastic substrates. There is lesselectromagnetic interference with control equipment than is produced bythe RF methods. Therefore, the DC magnetron sputtering process ispreferred for depositing the dielectric layers (either the intermediateor barrier layers, or both).

DC magnetron sputtering requires a sputtering target which iselectrically conductive. Assuming a target current of 50 mA/cm² and thatan induced through-target voltage drop greater than 50 volts isunacceptable, a maximum target material resistance of 1000 ohm/cm² isrequired. For a target thickness of 0.3 cm, the permissible resistivityis approximately 3300 ohm-cm or less.

A suitable material for nonreactive DC magnetron sputtering iselectrically conductive silicon carbide, available from the Standard OilCompany, Structural Ceramics Division, under the trademark Hexoloy. Agrade of the material, identified by this manufacturer as SG, isproduced by blending approximately 95 percent SiC and 5 percent graphitepowder by weight and adding suitable binders. The blended powders areextruded to form a sheet or rod, and then sintered in high temperaturegraphite furnaces. The final product consists of SiC grains in a matrixof electrically conductive porous graphite. The electrical conductivityat 20 degrees C. is between 0.2 and 300 ohm-cm, depending upon thedopants used.

Electrically conductive silicon carbide can be used as a DC sputteringtarget without bonding to a backing plate. The outer portion of thematerial has higher carbon content, which has been found to producesputtered films with carbon/silicon ratio strongly determined bysputtering time. A pre-sputtering procedure to clean the target is usedto improve the uniformity of the films produced.

The thin films produced with a silicon carbide and graphite sputtertarget have the chemical formula SiC_(x), wherein x, the molar ratio ofcarbon to silicon, is greater than 1. The carbon-rich silicon carbidefilms also have detectable concentrations of other elements, which arebelieved to be due to the binders used in a particular blend of powders.Generally speaking, the more electrically conductive the SiC/graphitesputtering target is, the higher the value of x in the deposited film.Excessively conductive blends produce films with large x values (e.g.,3.0 or more), but they lose transparency and thus are less acceptablefor magneto optic media applications.

FIG. 1 is a Auger Electron Spectroscopy (AES) spectrum of a transparentdielectric layer deposited on a substrate from a pre-sputtered target.The target material was Hexoloy SG SiC/graphite. The sputter targetsused were 0.25 inch (6.4 centimeter) thick by 12 inches (30.5centimeters) long by 5 inches (12.7 centimeters) wide. The spectrumindicates the presence of carbon, boron, silicon, nitrogen, and oxygenin detectable concentrations. Using the peak intensities and standardsensitivity factors known in the art, the atomic concentration of thedielectric was estimated as Si(35%)C(51%)B(7%)N(5%)O(2%), which yields avalue of x=(.51/.35)=1.47. The binders are the suspected source of boronand nitrogen, while the oxygen is believed to come from contamination ofthe vacuum chamber during sputtering. Because oxygen may react stronglywith one or more elements in the amorphous rare earth-transition metalalloy layer, terbium for example, the oxygen concentration should beminimized.

It is also desirable to perform the DC magnetron sputtering at low noblegas partial pressures, typically 0.01 torr or less. The resulting mediashow less decrease in coercivity with time (a measure of media storagestability) than media produced by sputtering at higher pressures.

The magneto optic recording media produced in the reduction of thisinvention to practice were substrate incident, quadrilayer media. Thesubstrates were glass or polycarbonate. The amorphous rareearth-transition metal (RE-TM) alloy used for the recording layercomprised approximately 69 atomic percent iron, 23 atomic percentterbium, and 8 atomic percent cobalt. The RE-TM layer was deposited bymagnetically enhanced triode sputtering. The reflector layer comprisedan aluminum-2% chromium alloy, and was deposited by nonreactive DCmagnetron sputtering. Both transparent dielectric layers were depositedby DC nonreactive magnetron sputtering. All layers were sputtered at1×10³ torr gauge pressure of ultra pure (99.999 percent minimum purity)argon.

The relative thicknesses of the magnetizable amorphous magneto opticfilm and the transparent dielectric intermediate layer in the trilayerconstruction, and the intermediate dielectric and barrier dielectriclayers and magnetizable amorphous film of the quadrilayer construction,are selected to yield a magneto optic angle of rotation exceeding thatof the medium without the added layers.

The effect of the thickness of the intermediate layer on mediacharacteristics was studied. Glass substrates were sputtered with 200angstrom thick barrier layers, 230 angstrom thick RE-TM layers, varyingthickness intermediate layers (in the range of 250 to 450 angstroms),and 1500 angstrom thick reflector layers. Both the barrier andintermediate layers were comprised of a SiC_(x) (x>1) transparentdielectric. Reflectivity was essentially linear with dielectric layerthickness, ranging from approximately 16 percent at 250 angstroms to 20percent at 450 angstroms. Carrier-to-noise ratio (1.4 micron bit size,30 kHz bandwidth) peaked at nearly 53 db at 300 angstroms. The rotationangle, θ, decreased essentially linearly from approximately 0.65 degreesat 250 angstroms thickness to approximately 0.20 degrees at 450angstroms thickness.

The effect of the thickness of the barrier layer on mediacharacteristics was also studied. Glass substrates were sputtered withvarying thickness barrier layers (in the range of 290 to 430 angstroms),250 angstrom thick RE-TM layers, 360 angstrom thick intermediate layers,and 800 angstrom thick reflector layers. Both the barrier andintermediate layers were comprised of a SiC_(x) (x>1) transparentdielectric. Reflectivity decreased rapidly with increased barrier layerthickness, ranging from approximately 28 percent at 290 angstroms to 18percent at 430 angstroms. Carrier-to-noise ratio (1.4 micron bit size,30kHz bandwidth) generally increased with increasing thickness, peakingat approximate 53 db at 430 angstroms. The rotation angle, θ, increasedessentially linearly from approximately 0.60 degrees at 290 angstromsthickness to approximately 0.85 degrees at 430 angstroms thickness.

The carrier-to-noise ratio (CNR) of media made in accordance with thisinvention is relatively large, examples having been measured atapproximately 50 db when measured with a laser diode at a wavelength ofabout 8300 angstroms. For representative embodiments of the invention,the threshold power for a laser diode in a write mode has been found tobe approximately 4 mW, which is acceptable because it is greater thanthe desired read laser power, typically 1-3 mW. Bit Error Rate (BER), ameasure of the amount of digital data lost due to degradation of theRE-TM layer with time, for certain of the inventive media is on theorder of 10⁻⁵.

EXAMPLE 1

To compare the invention to media prepared with the silicon suboxidecommon in the art, two series of quadrilayer media were prepared. Thesubstrates were identical and were made of injection moldedpolycarbonate. The silicon carbide dielectric layers were DC sputteredas described above, and the silicon suboxide layers were thermallyevaporated using techniques known in the art. Eight media wereconstructed with silicon suboxide (SiO_(y), y<2) dielectric layers, andfifteen media with the silicon carbide layers of this invention. Thefollowing table compares the average results of the two series. Datawere taken on tracks near the inside diameter of the media.Carrier-to-noise ratio (CNR) (1.4 micron bit size, 30 kHz bandwidth)data were taken with an 8300 angstrom laser diode at three differentwrite power levels, as shown. Bit error rate (BER) measurements weremade after 800 hours exposure to 80 degrees C. and 90 percent relativehumidity.

                  TABLE 1                                                         ______________________________________                                                                         Back-                                        Dielec-                                                                              Write                     ground                                       tric   Threshold CNR (db)        Noise                                        Material                                                                             (mW)      6 mW    7 mw  8 mW  (db)  BER                                ______________________________________                                        SiO.sub.y                                                                            3.8       50      51    51    -62   4 ×                                                                     10.sup.-5                          SiC.sub.x                                                                            3.2       48      50    50    -61   4 ×                                                                     10.sup.-5                          ______________________________________                                    

EXAMPLE 2

The resistance to thermal degradation over time of quadrilayer mediawith polycarbonate substrates and the carbon-rich silicon carbidedielectrics was also compared to that of media prepared with siliconsuboxide layers. For example, the carrier-to-noise level of run 422,made with SiC_(x), was approximately 95 percent of initial value afterexposure to an elevated temperature of 100 degrees C. for 1300 hoursbetween measurements (CNR was measured at room temperature). A similarcomparison for run 217, made with SiO_(y), showed a normalized CNR ofapproximately 95 percent of initial value after 220 hours exposure to115 degrees C.

EXAMPLE 3

Another measure of media stability is the change in coercivity of theRE-TM layer with time. Three samples, identified as runs 587, 595, and585, were constructed. They were trilayer construction comprising aglass slide substrate, a 200 angstrom SiC_(x) dielectric barrier layer,a 500 angstrom FeTbCo RE-TM layer, and a 200 angstrom SiC_(x) dielectricintermediate layer. The dielectric layers of different runs weresputtered at different argon partial pressures to test the effect ofthis parameter on media stability. Each dielectric layer of a given runwas sputtered at the same pressure. The media were exposed to 115degrees C. to accelerate their aging. Measurements of the coercivity ofthe RE-TM layer were made at room temperature after various times, asshown below.

                  TABLE 2                                                         ______________________________________                                                           Normalized Coercivity With                                      Sputter       Time (Hours) (1.0 at 0 hours)                              Run  Pressure (torr)                                                                             16     44     134   164                                    ______________________________________                                        587  1 × 10.sup.-2                                                                         .48    .39    .27   --                                     595  5 × 10.sup.-3                                                                         .55    --     --    .45                                    585  8 × 10.sup.-4                                                                         .73    .70    .70   --                                     ______________________________________                                    

Other samples from runs 587 and 585 were aged at 80 degrees C. and 90percent relative humidity for 230 hours. After the exposure, the mediawere inspected with a magneto optical looper, a device for measuringrotation angle as a function of applied magnetic field. The run 585medium exhibited the ability to magnetize a bit perpendicular to theplane of the RE-TM layer, and in the direction opposite that of theRE-TM layer adjacent to it. The run 587 medium did not exhibit thisability.

FIGS. 2 and 3 are transmission electron microscope (TEM) views ofsamples from runs 587 and 585, respectively, enlarged by factor of200,000. Layers 20 and 30 are the glass substrates, layers 21 and 31 arethe 200 angstrom SiC_(x) dielectric barrier layers, and layers 22 and 32are the RE-TM layers. The TEM sample preparation process removed the 200angstrom dielectric intermediate layer of each sample, as well as muchof the RE-TM layer 22 of the run 587 sample, leaving empty space at thelower portion of each figure. Inspection of the figures shows a moreuniform and dense structure in the dielectric layer 31 of run 585 thanis visible in the dielectric layer 21 of run 587. This experimentestablished that sputtering of the dielectric layer should beaccomplished at as low an argon partial pressure as possible, preferablybelow 5×10⁻³ torr.

EXAMPLE 4

To establish the effect of the carbon/silicon ratio, x, on the opticalproperties of the SiC_(x), a series of runs was made in which a SiC_(x)layer was deposited at 8×10⁻⁴ torr onto glass slides. The carbon orsilicon content of the films was adjusted by adding carbon or silicon,respectively, to the Hexoloy SG silicon carbide/graphite materialdescribed above. The x value was measured by Auger ElectronSpectroscopy. Optical properties of the dielectric layers were measuredusing a substrate incident 8300 angstrom laser. The results arepresented in Table 3.

                  TABLE 3                                                         ______________________________________                                        Run    R       T     A      t    n     k   x                                  ______________________________________                                        521    .42     .50   .04    474  3.0   .09 1.21                               506    .38     .53   .05    530  2.8   .09 1.57                               524    .34     .58   .04    480  2.7   .08 1.67                               626    .34     .49   .17    488  2.8   .20 2.10                               625    .28     .51   .21    456  2.7   .30 2.39                               ______________________________________                                         R is the reflection coefficient                                               T is the transmission coefficient                                             A is the absorption coefficient                                               (Note that R + T + A ≠ 1.0 due to measurement and rounding errors)      t is the thickness in angstroms                                               n is the index of refraction                                                  k is the extinction coefficient                                               (Note that n and k were calculated from R, T, A, and t using known            relationships)                                                                x is the carbon/silicon ratio                                            

As stated earlier, it is desirable to maintain the absorption of thedielectric layer below about 20 percent to ensure that the layer issufficiently transparent for use in a magneto optic medium. It is alsodesireable to maintain the index of refraction between about 2.0 and3.0, particularly in the case of an intermediate layer. This experimentestablished a preferred carbon/silicon ratio below approximately 2.4,more preferably between about 1.2 and about 2.0.

Although the media of this invention are erasable, they may be used inthe same applications as write-once or non-erasable media. A two-sidedmedium is also possible by combining two one-sided media through meansknown in the art. One may also groove the magneto optic recording mediato aid in locating recording tracks.

While certain representative embodiments and details have been shown toillustrate this invention, it will be apparent to those skilled in thisart that various changes and modifications may be made in this inventionwithout departing from its true spirit or scope, which is indicated bythe following claims.

I claim:
 1. A method of making a magneto-optic recording mediumcomprising a substrate, an amorphous magnetizable rare earth-transitionmetal alloy layer, a transparent dielectric layer on at least one sideof the rare earth-transition metal alloy layer, which method comprisesthe steps of:(A) providing the substrate, (B) depositing the amorphousmagnetizable rare-earth-transition metal alloy layer such that it iseffective in producing the magneto optic effect; (C) providing anelectrically conductive sputtering target comprising silicon and carbon;and (D) depositing, from the target provided, by DC magnetronsputtering, a dielectric layer comprising silicon carbide of the formulaSiC_(x), wherein x is greater than
 1. 2. The method of claim 1 whereinthe rare earth-transition metal alloy layer comprises iron, terbium, andcobalt.
 3. The method of claim 1 wherein x is greater than or equal to1.2.
 4. The method of claim 1 wherein the DC magnetron sputtering isperformed at a noble gas partial pressure less than 0.01 torr.
 5. Themethod of claim 1 wherein the electrically conductive sputtering targetcomprises silicon carbide and graphite.
 6. The method of claim 1 whereinthe magneto-optic recording medium further comprises a reflectivesurface located to reflect light through the magnetizable rare earthtransition metal alloy layer, and further comprising the step of:(E)producing the reflective surface such that it is effective in reflectingpolarized light transmitted through the rare earth-transition metalalloy layer.
 7. The method of claim 6 wherein step (E) comprisesdepositing a reflective layer.